1. Field of the Invention
The present invention relates generally to pin tumbler-type cylinder locks and associated keys. More particularly, this invention is directed to enhancing the security afforded by cylinder locks and especially to enhancing the ability of such locks to resist operation by a key obtained from an unauthorized source.
2. Description of the Prior Art
U.S. Pat. No. 4,823,575 discloses a novel cylinder lock and an associated key. The patented key, in addition to conventional bitting, is provided with a pair of projections on the opposite edges of the blade. These projections perform the dual function of operating an auxiliary pin tumbler stack, to permit rotation of the key and plug relative to the shell, and retaining the key in the keyway. The unique manner of implementation of the key retention function also reduces key and lock wear when force is imparted to the inserted key to move the door in which the lock is installed. U.S. Pat. No. 4,823,575 depicts a key wherein the bitting is in the form of depressions formed in the key blade, i.e., the patent depicts a “Kaba” type key. It will be understood by those skilled in the art, however, that the patented invention is equally applicable to a key wherein the bitting is, again by way of example only, in the form of conventional serrations in one or both edges of the key blade. Additionally, the patent depicts a reversible key, i.e., a key wherein the bitting is symmetrical and the two projections are of substantially the same size and shape. It will also be obvious to those skilled in the art that the patented invention is applicable to a key and lock combination where the pin tumbler arrangement is not symmetrical and where the projections serve separate and discrete functions, i.e., one projection operates the auxiliary pin tumbler stack while the other projection is intended for key retention only.
The lock and key of U.S. Pat. No. 4,823,575 have enjoyed significant commercial success. The patented combination, however, lacks a capability which many purchasers of high security locks desire. This capability is known in the art as “trapping”. With trapping capability, should an attempt be made to operate a lock with an unauthorized key, such key is captured in the lock if the attempt to rotate the plug relative to the shell proves successful. A “trapped” key may not be withdrawn from the keyway without disassembly of the lock and trapping will prevent further plug rotation.
U.S. Pat. No. 5,016,455 is directed to an improvement to the lock and associated key of U.S. Pat. No. 4,823,575 and particularly adds a trapping function. The cylinder lock is characterized by two separate parting lines, and particularly two shear lines located at different radial distances from the axis of the shell. The primary pin tumbler stacks are associated with a first of these parting lines while an auxiliary pin tumbler stack is associated with the second parting line. The second parting line is located at a greater radial distance from the cylinder lock axis than the first parting line. The shell of the lock has portions of at least two different diameters, commensurate with the two parting lines, and the plug has a shape which is, in two portions, complementary in shape to the shell. A circumferential retaining groove is formed in that portion of the shell which is associated with the auxiliary pin tumbler stack. A longitudinal slot or recess having the same depth as the groove communicates between the forward end of the shell and the groove. An associated key has, in addition to conventional bitting for operating the primary pin tumbler stacks, a pair of projections extending from the opposed edges of the key blade. These projections are, at least in part, in alignment. A first projection causes translation of the auxiliary pin tumbler stack to permit rotation of the plug relative to the shell. The second projection is longer than the first projections and will pass along the longitudinal slot to engage the groove to retain the key in the plug subsequent to rotation thereof. The second projection also cooperates with the drive pin of the auxiliary pin tumbler stack, upon rotation of the plug by 180 degrees, to prevent the driver pin from engaging the keyway in the plug, such engagement trapping the key and disabling the lock.